IR-spectroscopy of CO adsorption on mixed-terminated ZnO surfaces

The adsorption of CO on two different mixed-terminated, single-crystal surfaces of zinc oxide, ZnO(10 (1) over bar0) and ZnO(11 (2) over bar0), was investigated by employing infrared reflection absorption spectroscopy (IRRAS) using p- and s-polarized light. For both surfaces, one negative CO band was observed only for p-polarized light, indicating that CO is bound to the surface Zn2+ sites in a nearly upright geometry. In addition, we observed a substantial coverage-induced frequency shift for CO adsorption on both ZnO surfaces. For ZnO(10 (1) over bar0), the v(C-O) band shows a red shift of 16 cm(-1) when increasing the CO coverage from 0.5 ML (2185 cm(-1)) to 1 ML (2169 cm(-1)). On the ZnO(11 (2) over bar0) surface, for isolated CO molecules, a frequency of 2192 cm(-1) was detected. The CO band shifts to 2170 cm(-1) at full monolayer. On both surfaces, the coverage-dependent frequency shift is attributed to a combination of dynamic and substrate-mediated static adsorbate-adsorbate interactions. (C) 2016 Elsevier B.V. All rights reserved.